1. Field of the Invention
This invention relates to high-purity .alpha.-type silicon nitride comprised of a fine granular crystal, and also to a process for the preparation thereof.
2. Description of the Prior Art
As is well-known, silicon nitride or sintered silicon nitride have advantageous properties over conventional ceramic products, such as (1) high mechanical strength and hardness and high strength at high temperatures, (2) high resistance to thermal shocks and high refractoriness, (3) relatively high thermal conductivity, (4) very small coefficient of thermal expansion, (5) good chemical stability and high corrosion resistance and (6) high electric insulation.
By virtue of these excellent properties, silicon nitride and sintered silicon nitride have heretofore been used as high-quality refractories, refractory materials, wear-resistant materials and electric insulators in the fields of the metal refining industry, the ceramic industry and the machine industry. Recently, silicon nitride and sintered silicon nitride have attracted attention as high-temperature materials for gas turbines, for which high strength, high resistance to thermal shocks and high resistance to mechanical shocks are required over a broad temperature range.
When sintered silicon nitride is put into practical use as a high-temperature high-stress material, high physical and chemical stability is strictly required for this material. Properties of this material, especially thermal and mechanical properties, are greatly influenced by the purity, crystal form, particle size and particle shape of silicon nitride used as the starting material to be sinterd and also by the kind of the starting material used for production of silicon nitride. When this material is used in the above-mentioned fields, for example, as a structural material for a gas turbine, it is required that silicon nitride should be in the form of a powder of high-purity .alpha.-type silicon nitride comprised of a fine granular crystal. However, powdery silicon nitride satisfying these requirements has not been developed, and it has been considered very difficult to produce such a powdery silicon nitride.
As the conventional method for preparing silicon nitride powder, there can be mentioned a so-called silica reducing method (1) in which a silica powder and a graphite powder are heated together in a nitrogen atmosphere, whereby the silica powder is reduced by the graphite powder to form an active silicon-containing vapor and, then, this vapor is reacted with nitrogen. This method, however, is defective in that both the silica powder and the graphite powder used as the starting materials must be highly refined, and the obtained product is a mixture of .alpha.-type silicon nitride, .beta.-type silicon nitride, silicon oxide-nitride and silicon carbide, and contains a large amount of oxygen but has a low nitrogen content. In other words, it is difficult to obtain high-purity .alpha.-type silicon nitride powder. As another conventional method, there can be mentioned a direct nitriding method (2) in which a metallic silicon powder is first formed and it is directly nitrided in a nitrogen or ammonia stream under heating at a temperature lower than 1,500.degree. C. while controlling the nitrogen gas pressure. Silicon nitride obtained according to this method contains a large amount of .beta.-type silicon nitride, and is generally not in a fine powder form. Furthermore, it is necessary to pulverize the reaction product for a very long time. Accordingly, incorporation of impurities at the pulverizing step cannot be avoided, and the resulting silicon nitride powder is not suitable as the starting material for the manufacture of sintered silicon nitride having a high density and a high strength. As still another conventional method, there can be mentioned a method (3) in which silicon nitride is formed by a high-temperature vapor phase reaction between a silicon halide and ammonia. This method is advantageous in that .alpha.-type silicon nitride having a relatively high purity can be obtained, and this method is suitable for manufacture of a thin film of silicon nitride. However, this method is not suitable for manufacture of a starting powder for sintered silicon nitride. According to still another conventional method (4) comprising thermally decomposing silicon imide, .alpha.-type silicon nitride can easily be obtained, but this method is defective in that when an oxide or a non-oxide material forming a strong oxide film is used as the furnace-forming material, the obtained silicon nitride powder has a fibrous or needle-like form, and this powder is poor in shapeability at the step of forming a sintered product and uniform and homogeneous mixing of this powder with a sintereing aid is difficult.